Post-Spectrometer Instrumentation for STEM
A charge-coupled device (CCD) is being used for parallel recording on the EELS spectrometer of the V-G HB501 STEM in the Cavendish. Three quadrupole lenses are provided for focussing the EELS spectrum and, unlike in some other similar systems, the spectrometer slit has been retained. This was done so that the spectrometer could still be used for serial acquisition of EELS spectra and for energy-filtered imaging, although it would be preferable, and was envisaged as a later development, to have the first quadrupole positioned before the dispersion plane of the spectrometer; the slit would have been simulated by a suitably masked scintillator for the bright field detector.However it has proved advantageous to keep the slit so that microdiffraction patterns can be imaged through the spectrometer. The recording of high quality diffraction patterns in the HB501 STEM has not been straightforward: Grigson recording is far too slow, and although the standard microscope is provided with a phosphor screen and TV and film cameras, the speed and resolution are rather poor.